236 THE AMERICAN MIDLAND NATURALIST 



mouth. The usual tendency is for the nacreous color to considerably 

 lighten or become bluish. 



3. The shells of L. Erie have a greater proportion of blues among 

 them than the corresponding shells in the Upper Ohio Drainage, 

 and Maumee Drainages. The shells of Conneaut and Chautauqua 

 Lakes have the same relation. 



Other conclusions, not however as completely substantiated as 

 those given above, but still so evident from the present data as to 

 deserve mention are: 



I Each drainage leaves its own imprint on the shells collected 

 from it in the form at least of an associated peculiar tone of nacre 

 color. (This has already been observed with regard to other physical 

 characters). While the same "relative colors" may be present in 

 different drainages, these are usually distinguished when necessary 

 by varying proportions of other colors. 



2. As a rule, the color distinction may be carried so far as to say 

 tentatively at least, that certain shades of nacre color are charac- 

 teristic of certain localities in a given body of water. While as ob- 

 served in the chart, this is best observed in the case of Presque Isle 

 shells, ample verification is obtainable in shells from the Upper 

 Ohio Drainage. 



3. So far as we may consider results obtained from a small 

 number of shells in many cases, sex correlative coloration seems to 

 be associated with nacre color. 



VIII. — Suggestions as to Causes oe Facts. 



Introductory remarks embody the writers' comments on the first 

 of these conclusions. Any plausible explanation of the second would 

 seem to be found in the physical and chemical conditions under 

 which the shells live. A summary of the physical and chemical 

 conditions present in the Upper Ohio Drainage and 'L. Erie has 

 been given. Perhaps that physical condition most applying to the 

 problem of nacreous color is the warmer temperature of the water 

 in the former, for it has been seen that as a rule. Upper Ohio shells 

 possess more pigment than those of Iv. Erie. Pigment is the result 

 of chemical reaction, and chemical reactions in general are increased 

 by the amount of heat. 



Wilson and Clark, (18, 19) were inclined to associate with the 

 fading out of the nacreous color of Elliptio an abundance of lime, 

 and, (conversely), the want of humic acid in the lower stretches of 



